Keeping our Rivers Clean at the Sewage Treatment Works
Meera - This week, for some true scientific glamour, I have come along to the Cambridge sewage treatment facility located on Cowley Road in Cambridge. With me is Sarah Rowland and Nicola Marvin, both from Anglian Water, and they're going to take me through the various steps, processes and biological treatments that take place here in order to provide Cambridge with clean water. So, first of all Sarah, tell me a bit about this site.
Sarah - Well, sewage treatment has been carried out on this site since 1895. Before that, and we're talking back in Victorian times, everything used to go straight out into the river Cam untreated. Of course, you had dysentery, cholera, the plague, all sorts of nasty things, and Cambridge wasn't a very pleasant place to live in. But come 1895, the Victorians got a real hang of the idea of sewage treatment and they started pumping it here into what was called a sewage farm. They used to put it on the land and literally grow crops on it. We do use natural processes still today, but it's a little bit more sophisticated, and we're treating for a population of about 155,000 people every single day. That's the equivalent of about 5 million toilet flushes every single day, coming in here to be treated before it goes out clean into the river Cam.
Meera - We're currently standing at the inlet works which is very high up and I've got a good view of the entire sewage works. It's a big site, 200 acres. So what are the various stages that take place here?
Sarah - Well the first thing we do is screen out all the rags, all the disposable things that people have put down their loo that should be going in the dustbin. Items such as sanitary towels, condoms, anything that people think that they can get down the toilet and seem to like trying to do so, should be going in the bin. We take those out first, and then what we'll start doing is separating out solids from water. We've got some settlement tanks where everything heavy settles to the bottom and that goes for separate treatment. If you follow the effluent, the runny side of things, it gets digested using biological processes, using microbes, using bacteria that take out different elements of the sewage. We then settle it again to separate out the microbes or bacteria from the liquid. We put it through sand filters on the far side of the site which is a final polishing, just to make sure that the effluent has got a crystal clear quality to it, and then it finally goes out in the river Cam.
Meera - Thanks, Sarah. Now, Nicola, you're one of the service delivery scientists here at the sewage works. You're going to take me over now to one of the trickling filters which is where the first stage of the bacterial processes takes place.
Nicola - That's right, yes. Up until now, it's just been all physical processes, so that's where all the guts of the treatment happens.
Meera - So we've come alongside one of the trickling filters now. Their name is pretty much what they do, there's a large pipe moving along the top of a large bed of stones and it's trickling water onto the stones.
Nicola - That's right. The water comes through from the top surface and it's stones all the way down to through the filter bed. And these are probably about 1.8 to 2 metres deep. The water works its way down through and on the stones, we've actually created an environment that the biomass likes to live in. The biomass is bacteria. It's made up of hundreds or thousands of different types of bacteria and animals that eat the bacteria. It's a whole ecosystem on its own. So things like the carbonaceous treatment happens at the top, so basically, that's where the guts of the carbon is removed from the water. As this works its way down through the bed, the nitrifiers then proliferate. They love it down at the bottom because they get to eat all the ammonia that comes from urea. Urine breaks down to urea which then breaks down to ammonia. So things like nitrosomas and little bacteria like that, they live down near the bottom.
Meera - So, looking at the bottom and the water coming out now, what kind of things do we have left in here?
Nicola - As the water moves down through the bed, the natural control mechanism for the bacteria in the bed itself tends to get knocked off. By the time it comes out the bottom, we can see that it looks quite dirty, but actually, it's fully treated. The bacteria have done most of the treatment job here. They've taken out things like ammonia that will cause impact on the water course by causing eutrophication later on. If there's too much ammonia and phosphorus in the environment, you get algal blooms and things like that.
Meera - What then happens to just finally give it a good clean?
Nicola - Straight after the biological treatment, they all have their own settlement tanks and that's where we take the biomass, the bacteria, out of the system. We're separating basically the solids from the liquid.
Meera - What are the levels that you consider acceptable then for the water to contain of particular contaminants when it's released back into the Cam?
Nicola - Now the standards here at Cambridge are 20 [mg/l] for total suspended solids - that's the amount of 'bits' basically left in the water, 15 [mg/l] for BOD, Biochemical Oxygen Demand, and 5 [mg/l] for ammonia.
Meera - As you mentioned, one of the first stages is primary settlement where some of the more solid waste is settled out and removed. Various biological processes also take place with these solids as well.
Nicola - That's right. They have to go through a digestion stage so that we can actually fully utilise the sludge that's produced and actually make it reusable. So let's take a walk down to the far end of the site where the digesters are.
Meera - We are now surrounded by lots and lots of tanks. They're bigger than two or three stories high, some of them up to six stories high. So, what is taking place in all of these tanks?
Nicola - Well, we have to blend all the sludge that we create on-site, the indigenous sludge. So, the primary sludge, solids that we take out of the final settlement tanks from the filters, and then those in turn are blended into the tank that we're standing next to which is a feed tank for the monsal plant and the monsal plant is made up of six large green tanks. The first stage of those is at 42 degrees so, mesophylic digestion. Digestion has to happen without air, so this part of the process is all anaerobic. There's three stages: there's hydrolysis, breaking down the long carbon compounds into smaller chains, then there's acetogenesis which is where the bacteria have changed it into a food source for other bacteria, for the methanogens. Then the methanogens will actually produce methane. We start off at 42 degrees and then we push it through the rest of the monsal plant at 55 which kills all the pathogens, because sewage has got nasties in it. Viruses, bacteria that we don't want, that cause diseases, so we have to kill them off. We then push through into the two big digesters, the six-story high ones where we gather most of the methane that's produced by the sludge process. Before it can get to their final stage, we actually have to do what we call de-watering. The sludge comes out as a lovely, crumbly, what we call sludge cake or biosolids.
Meera - And this sludge cake is then used on agricultural land.
Nicola - That's right. The farmers love it. It's a really good soil conditioner and it's got phosphorus and nitrogen in it.
Meera - The methane that's produced is also used to help power the site?
Nicola - That's right. We run a combined heat and power unit and some boilers off the methane gas that's produced, and we can actually back feed into the national grid.
Meera - Well now, having understood what happens to the solids that's removed and how the water's treated, let's go join Sarah who's on the furthest part of the site, where the effluent that's actually created enters the river Cam and we can have a look at how clean it is.
Meera - Right. Hello again, Sarah.
Sarah - Hello.
Meera - So, we're now by the final effluent that actually enters the river Cam. First of all, I have to comment on the fact that it smells actually quite nice here!
Sarah - Well if you think this is our finished product, we don't want this to have any nasty effect on the river.
Meera - So we did see the water at its original murky stage. Now, how long does it take for it to become this clean?
Sarah - From coming through the front door, to going out of the backdoor, shall we say, it takes anywhere between 8 and 10 hours. We've literally squeezed every last drop out of it. We've got rid of all the pollutants that we can, we've harvested all the energy in terms of methane, we've harvested a valuable soil conditioner for the farmers, and we're putting back that final bit, the effluent, in a nice clean state, back into the river Cam. We've come on a long way since the Victorians, let's face it!